Rechargeable batteries are used in almost every aspect of daily life. A wide variety of industrial, commercial and consumer applications exist. Larger capacity battery uses include such applications as fork lifts, golf carts, uninterruptable power supplies for protection of electronic data storage, and even energy storage for power production facilities. When electric vehicles are manufactured in mass, demand for low weight, high charge capacity batteries will be greater than ever before. Indeed, to make mass use of electric vehicles economically feasible, very high specific capacity may be critically necessary.
In electric vehicles, weight is a significant factor. Because a large component of the total weight of the vehicle is the weight of the batteries, reducing the weight of the cells is a significant consideration in designing batteries to power electric vehicles.
The 1998 California Clean Air Act has posed an exceptional challenge on battery scientists and engineers to develop an improved battery that can support the commercialization of electric vehicles (EV). Needless to say, the law has not changed the reality of battery technology. In over 100 years of rechargeable battery usage, two chemistries namely: Pb-PbO.sub.2 (known as lead-acid battery) and Cd-NiOOH (known as Ni-Cd battery) have dominate with more than 90% of the market. Neither of the two are likely to fulfill the utopian goals of powering an electric car that will match the range, economy, and performance of an internal combustion engine vehicle. Therefore, battery scientists and engineers are forced to study new battery chemistries.
In addition to industrial, commercial and other large scale uses of batteries, there are literally thousands of consumer applications of rechargeable batteries. A rechargeable electrochemical cell is ideally suited to serve as a portable power source due to its small size, light weight, high power capacity and long operating life. A rechargeable cell may operate as an "install and forget" power source. With the exception of periodic charging, such a rechargeable cell typically performs without attention and rarely becomes the limiting factor in the life of the device it powers.
Present rechargeable battery systems can be classified into two groups those employing liquid electrolytes and those employing solid electrolytes. Liquid electrolyte systems have been around for many decades and are the most well known to the general public. Examples of liquid electrolyte rechargeable battery systems include lead-acid, nickel cadmium, and the more recent nickel-metal hydride systems.
A more recent advancement is the solid electrolyte rechargeable battery systems. The solid electrolyte devices have several distinct advantages over those based on liquid electrolytes. These include (1) the capability of pressure-packaging or hard encapsulation to yield extremely rugged assemblies, (2) the extension of the operating temperature range since the freezing and/or boiling-off of the liquid phase, which drastically affect the device performance when employing liquid electrolytes are no longer a consideration, (3) solid electrolyte devices are truly leak-proof, (4) they have long shelf life due to the prevention of the corrosion of electrodes and of loss of solvent by drying out which occur when using liquid electrolytes, (5) solid electrolytes permit micro-miniaturization, and (6) the do not require heavy, rigid battery cases which are essentially "dead weight" because they provide no additional capacity to the battery but must be included in the total weight thereof.
All of the above considerations have led to a growing use of solid electrolytes. Solid state batteries and timers are already available which employ the solid electrolyte as a cylindrical pellet with suitable electrodes on either side. However, this kind of geometry leads to somewhat poor solid-solid contacts and these devices tend to have high internal resistances and polarization losses. These problems have been overcome by the use of thin films as the electrolytes, since thin films deposited on top of each other have excellent contacts and should also be able to withstand shocks, acceleration forces and spin rates without undue damage.
In forming such a battery system, a solid ion conductor (i.e. solid electrolyte) for moving ions within the system is required. A solid electrolyte is classified by its type of movable ion, such as Li.sup.+ -conductive solid electrolyte, Ag.sup.+ -conductive solid electrolyte, Cu.sup.+ -conductive solid electrolyte, H.sup.+ -conductive solid electrolyte, etc. A solid electrochemical element is constituted by combining one of these solid electrolytes with an appropriate electrode material. Several solid electrolytes are known to exhibit good ionic conductivity, some of which exist in the form of thin films. Oxide ion conductors such as zirconia are operated at high temperatures due to their low conductivity at ambient temperatures. Chloride ion conductors such as PbCl.sub.2 and BaCl.sub.2 have similar temperature restrictions. Silver ion such as AgBr, AgCl, and AgI also show low room temperature ionic conductivity.
Of the thin-film, solid state battery systems, lithium-polymer batteries have received the most widespread interest. Reports in 1979 that lithiated poly-ethylene-oxide (PEO) possesses lithium ion conductivity raised the expectations for a solid state battery employing PEO as solid electrolyte. Indeed, if PEO, or other polymers, were a true solid electrolyte with practical ionic conductivities and a cationic transfer number of 1, a stable interface with the lithium electrode and good charging uniformity could be realized. The expectations, no doubt, were stimulated by the relative success of the true solid electrolyte "B" Alumina, in the Sodium Sulphur battery.
More recently, several researchers proposed the use of "plasticized polymers" to enhance conductivity at room temperature. Although the term "plasticized polymers" is the correct material science terminology for the materials, they are in effect no different than a battery separator filled with organic solvent and electrolyte. In this case, we are back to liquid filled systems with all the old unsolved fundamental problems and several new ones.
Solid electrolytes consist of solid atomic structures which selectively conduct a specific ion through a network of sites in a two or three dimensional matrix. If the activation energy for mobility is sufficiently low, the solid electrolyte can serve as both the separator and electrolyte in a battery. This can allow one to fabricate an all solid state cell.
An important aspect of such electrolytes is that they selectively conduct only one type of ion. If that ion features reversible electrochemistry with both the positive and negative electrode of the battery, and if the solid electrolyte itself is inert to the electrodes, the cell will enjoy a uniform and reversible electrochemistry with no composition change and no passivation or side reactions.
While true solid electrolyte lithium conductors would not exhibit the inherent problems of Li-polymer systems described herein below, all polymer electrolytes reported to date are not true solid electrolytes. The conductivity occurs in an amorphous zone that conducts anions better than it conducts lithium ions (the transfer number of lithium is less than 0.5). As such, ion concentrations in the electrode surface are variable and irreversible reactions between the anion and the lithium electrodes do occur. The combination of the two effects brings about partial passivation of the lithium surface with non uniform dendritic plating on charge. Additionally, the conductivity of the polymer electrolyte is too low, typically two to four orders of magnitude lower than that of aqueous electrolyte. Also, the electrode area required for a 20 kwh battery is 42 m.sup.2 for Ni-Cd batteries and is 1610 m.sup.2 for Li-Polymer batteries. This data clearly conveys that in order to deliver acceptable power levels for EV applications, lithium polymer batteries will require nearly two orders of magnitude, larger electrode area per ampere hour than a higher power density Ni-Cd battery. Given that electrode processing is the most expensive component in battery production and that the cost of electrode processing is nearly linear with electrode area, the cost implications of the design are astonishing.
In addition to cost, safety of Li batteries, particularly liquid electrolyte systems, is always a problem. The single most important reason rechargeable lithium batteries have not been successful in the market place is their poor safety record. Most research groups that have worked on rechargeable lithium cells have "personally experienced" explosions, and explosions have occurred in the field. The problem can be diagnosed as follows: 1) lithium plating is dendritic, 2) dendrites eventually short through the separator, 3) shorted cells heat up during charging, 4) shorted cells will go into reversal during full battery discharge, 5) low capacity cells will go into reversal during full battery discharge, 6) in reversal, lithium is likely to plate on the cathode which can cause direct chemical reaction between cathode material and lithium, 7) processes 3 and 6 can generate enough heat to melt lithium (165 Centigrade), and 8) molten lithium is an extremely strong reducing agent which will react with most organic and inorganic materials. An explosion could occur depending on: (a) the amount of lithium in the cell, (b) the surface to volume aspect ratio of the cell, (c) the reactivity of the other cell components to lithium, (d) the vapor pressure of the products, and (e) the vent design.
Battery design should be aimed at minimizing the risk of lithium melt down. Given that it is extremely unlikely that lithium melt down can be completely avoided in mass usage of large rechargeable lithium batteries, it is essential to guarantee non explosion when the melt down does occur. Dry polymer electrolyte offers some improvement with regard to exposition when compared to high vapor pressure liquid electrolyte. However, that improvement is counteracted by the need for a very thin separator. Overall, the likelihood of ensuring explosion free melt downs in large cells and batteries is diminutive.
Cells utilizing polymer electrolytes that contain organic solvents, are as likely to be explosive as cells with standard (polymeric) separator and liquid electrolytes. In this case, depending on cell design, common experience places the explosion threshold in the 0.5 to 5 Ah size range; two orders of magnitude smaller than what is required for an EV battery. It should be noted that a cycled lithium electrode is more prone to explosion than a fresh uncycled one. While this fact has been known for quire some tine, lithium polymer battery developers have shied away from publishing safety test data on cycled cells.
In spite of its safety problems, there is a continued interest in lithium batteries because of their purportedly high power density. This feature makes rechargeable lithium batteries attractive. Theoretical energy densities of most rechargeable lithium chemistries are two and a half to three times higher than that of Pb-Acid and Ni-Cd batteries. Indeed, liquid electrolyte rechargeable lithium batteries could be made to deliver up to 150 Wh/Kg and 200 Wh/liter. This is about three times higher than the practical gravimetric energy density delivered by the best Ni-Cd batteries and four times higher than the practical gravimetric energy density delivered by the best Pb-Acid batteries. However, the design of the lithium polymer batteries, driven by the poor conductivity of the polymer electrolyte, is very volume inefficient. Specifically, the separator occupies 30% of the stack volume, carbon is added to the positive electrode in concentration of up to 30% and the positive electrode utilization is poor. Thus, the practical energy density is likely to be considerably lower than of what can be achieved with liquid electrolyte. Estimated deliverable energy density of lithium polymer batteries is 15-20% of the theoretical energy density. This translates to (using 485 Wh/Kg as theoretical maximum) approximately 70 to 100 Wh/Kg at best. Most likely, compromises that will have to be made to improve manufacturability, safety and cycle life beyond the current laboratory state-of-the-an technology. This will have the effect to reduce the practical energy density to even below the values proposed above. The power capability of a battery depends upon the physical and chemical properties of the cell components as well as the cell design. Lithium polymer battery developers are trying to counteract the poor inherent conductivity of the polymer electrolytes by reducing the electrode and separator thickness. Because practical manufacturing reality is likely to impose increases in the electrolyte thickness from approximately 2 to 4 mil, the power deliverable by the cell is likely to drop by 30 to 50%.
An area that requires closer attention is power degradation over life. The main degradation mechanism of the cell involves irreversible reactions between lithium and electrolyte. This reduces the conductivity of the electrolyte as well as increases the impedance of the Lithium electrode due to the formation of passive films; both effects reduce the deliverable power from the battery. Because the cycle life of the lithium polymer battery is short, significant degradation in power is likely to occur in less than 100 cycles.
Other problems arise from real life usage and requirements placed upon battery systems. Traction batteries are assembled from a string of individual cells connected in series. During both charge and discharge, the same amount of current will pass through all the cells. In practical manufacturing and usage, it is impossible to keep all cells at exactly the same state of charge. This forces a weak cell in a battery to go into reverse during deep discharge and some cells to go into overcharge during full charge. For a battery to operate at deep discharge cycles, it is essential that individual cells tolerate reverse or overcharge without damage or safety implications.
Lithium batteries are very poor in this respect. Over discharge will result in plating lithium on the positive electrode which can result in a spontaneous chemical reaction with severe safety implications. Overcharge is likely to result in electrolyte degradation that can generate some volatile gasses as well as increase cell impedance. These problems are particularly severe for lithium cells because: 1) degradation occurs during cycle life, therefore, even if initial capacities are matched very closely, it is unreasonable to expect that the degradation rate will be identical for all cells, 2) the cells tend to develop soft or hard shorts, thereby making it impossible to maintain the cells at the same state of charge at all times, and 3) cell capacity is dependent on temperature, therefore cells that are physically cooler due to their location will deliver less capacity than others. These conditions make the likelihood of cell reversal, relatively early in the life of the battery, very high. Of course, cell reversal is likely to result in venting and or explosion.
It has been propose to install individual diode protection for all cells which could be an expensive, although practical, solution for a portable low watt-hour battery. The increased cost and reduced reliability associated with this solution makes this very undesirable for an EV battery. Plus, the inherent lack of overcharge and over discharge capability eliminates any possibility of ever developing a rechargeable lithium-polymer battery of a bipolar design.
An additional problem with the commercialization of Li-polymer batteries is their high cost. It is difficult to assess the cost, although clearly, processing cost per watt-hour should be much higher than that of traditional batteries. Raw material costs are clearly higher than Pb-Acid, although, it may be similar to Ni-Cd. The cost of raw material will rise due to high purity requirements. There are convincing reasons to expect that lithium polymer batteries, if ever made commercially, will be considerably more expensive than Ni-Cd batteries considering that: 1) primary Li-MnO.sub.2 cells, which are in mass production, are still more expensive than Ni-Cd cells, 2) the purity requirements for a secondary cell are much higher than that of a primary cell, and 3) the electrode area per watt-hour of a lithium polymer secondary battery will be approximately an order of magnitude larger than that of a primary Li-MnO.sub.2 battery.
Even more problematic than the cost factor is the low cycle life of the lithium polymer batteries, which is particularly important in EV applications. Small rechargeable lithium batteries employing organic liquid electrolyte have delivered 100 to 400 cycles in laboratory tests. It is anticipated that lithium polymer electrolyte batteries of the same size could be made to deliver a comparable number of cycles. However, all the data published to date on lithium polymer batteries was run on cells with a very large amount of excess lithium, therefore, no conclusion can be drawn at this stage.
The cycle life of a large multi cell battery is likely to be considerably lower than that of a small two-cell battery. Additional reduction of the expected cycle life results from consideration of the fact that the battery will be limited by the weakest cell, and as previously mentioned, the likelihood of temperature or electrical imbalance is high. Further, power may degrade faster than capacity, so cycle life could become limited due to an unacceptable drop in power. Therefore, it is probably a fair assumption that if a full size battery was built at today's state-of-the-art technology, it could possibly make 100 cycles or so, which is about an order of magnitude short of what is required for an EV.
Therefore, since lithium-polymer batteries will be inadequate to meet today's requirements for a universally acceptable, thin-film, solid state rechargeable secondary battery system, other solid state systems need to be developed. Additionally, there is a need for an apparatus to easily and economically fabricate such batteries.